Low drop-out (LDO) voltage regulator circuit

ABSTRACT

A low drop-out (LDO) voltage regulator circuit includes a power transistor having a control terminal configured to receive a control signal and an output terminal coupled to an output node. A current regulation loop senses current flowing through the power transistor and modulates the control signal to cause the power transistor to output a constant current to the output node. A voltage regulation loop senses voltage at the output node and modulates the control signal to cause the power transistor to deliver current to the output node so that an output voltage at the output node is regulated. The current regulation loop includes a bipolar transistor connected to the control terminal of the power transistor, where a base terminal of the bipolar transistor is driven by a signal dependent on a difference between the sensed current flowing through the power transistor and a reference.

TECHNICAL FIELD

Embodiments generally relate to a low drop-out (LDO) voltage regulatorcircuit and, in particular, to an LDO voltage regulator circuit that isadaptable to a wide range of charging applications.

BACKGROUND

With reference to FIG. 1, a conventional multi-mode low drop-out (LDO)voltage regulator circuit 10 is configured to support operation in onemode to enable a voltage regulation loop 14 to regulate the delivery ofa voltage at a load 16 and in another mode to enable a currentregulation loop 18 to regulate the delivery of a current to the load 16.The output voltage Vout from the circuit 10 at the load 16 is sensedusing a resistive voltage divider formed by the series connection ofresistor R1 and resistor R2. The tap node of the resistive voltagedivider generates a feedback voltage Vfb. A voltage comparator circuit20 compares the feedback voltage Vfb to an output reference voltageVref_out to generate an enable signal En_V for controlling the enablingof operation of the voltage regulation loop 14. The enable signal En_Vis deasserted (for example, logic low) by the voltage comparator circuit20, and the voltage regulation loop 14 is disabled, if the feedbackvoltage Vfb is less than the output reference voltage Vref_out.Conversely, the enable signal En_V is asserted (for example, logic high)by the voltage comparator circuit 20, and the voltage regulation loop 14is enabled, if the feedback voltage Vfb is greater than the outputreference voltage Vref_out. A logic inverter circuit 22 inverts thelogic state of the enable signal En_V to generate an enable signal En_Cfor controlling the enabling of operation of the current regulation loop18. The enable signal En_C is asserted (for example, logic high) by thelogic inverter circuit 22, and the current regulation loop 18 isenabled, if the feedback voltage Vfb is less than the output referencevoltage Vref_out. Conversely, the enable signal En_C is deasserted (forexample, logic low) by the logic inverter circuit 22, and the currentregulation loop 14 is disabled, if the feedback voltage Vfb is greaterthan the output reference voltage Vref_out.

A power MOSFET device 24 has a source-drain current path coupled betweenan input voltage (Vin) node and the output voltage (Vout) node and iscontrolled to source a current to the output node. More specifically,the drain terminal of transistor 24 is coupled to the input voltage(Vin) node and a source terminal of transistor 24 is coupled to theoutput voltage (Vout) node. The gate terminal of the transistor 24 isconfigured to receive a gate voltage Vgate.

The voltage regulation loop 14, when enabled by the enable signal En_V,controls the transistor 24 to deliver current to the load 16 in order toregulate the output voltage Vout at a level where the feedback voltageVfb is substantially equal to a voltage regulation reference voltageVref_vol (where Vref_out<Vref_vol). The gate voltage Vgate driving thegate terminal of the transistor 24 is generated by a differentialvoltage amplifier 30 having a first input configured to receive thefeedback voltage Vfb and a second input configured to receive thevoltage regulation reference voltage Vref_vol.

The current regulation loop 18, when enabled by the enable signal En_C,controls the transistor 24 to deliver a regulated constant current tothe load 16. A current sensing circuit 32 senses the magnitude of thecurrent flowing in the source-drain path of the transistor 24 andgenerates a feedback voltage Cfb indicative of that sensed currentmagnitude. The gate voltage Vgate driving the gate terminal of thetransistor 24 is generated by a differential voltage amplifier 34 havinga first input configured to receive the feedback voltage Cfb and asecond input configured to receive a current regulation referencevoltage Vref_cur. The differential voltage amplifier 34 drives thetransistor 24 so that the feedback voltage Cfb is substantially equal tothe current regulation reference voltage Vref_cur.

The voltage comparator circuit 20 and logic inverter circuit 22 exercisea digitally controlled transition between the constant voltage mode(i.e., when the voltage regulation loop 14 is enabled) and the constantcurrent mode (i.e., when current regulation loop 18 is enabled). It isnoted that undesirable current and voltage glitches can occur whenswitching from the constant current mode to the constant voltage mode.These glitches arise as a consequence of the digitally controlled modeswitching operation. In some applications of the LDO circuit 10, theseglitches present a serious concern. For example, if the load 16 ishighly capacitive (such as in the range of 1-10 mF or even higher up to1F, sometimes referred to in the art as a “super-cap” load as might beassociated with the load 116 being a rechargeable battery that ischarged by the LDO circuit), the magnitude of the current glitch canquickly reach a level of tens of Amps before transient protectioncircuitry has time to react. Glitches of this magnitude can have anadverse impact on the load circuit (for example, damaging the battery).

The reference by Chia-Hsiang Lin, et al., “A Li-Ion Battery Charger WithSmooth Control Circuit and Built-In Resistance Compensator for AchievingStable and Fast Charging”, IEEE Trans. on Circuits and Systems 57-I(2):506-517 (2010), incorporated by reference, teaches an analog switchingtechnique for changing between constant voltage and constant chargingmodes. However, it is noted that during mode transition where the outputvoltage is approaching the targeted value, the voltage regulation looptends to want to increase the gate voltage whereas the currentregulation loop tends to want to decrease the gate voltage. Because theoutput impedance of the differential amplifier in the current regulationloop is low, the current regulation loop is incapable of immediatelyturning off. The push-pull between the two regulation loops inducesringing/oscillations for a time period before the voltage regulationloop fully takes over control. It is worthwhile to note that inducedringing/oscillations will be exacerbated in the case where the load ishighly capacitive (such as with a “super-cap” implementation asdescribed above). The reason for this is that the capacitive load willprolong the duration of time over which the push-pull between the tworegulation loops occurs.

There is accordingly a need in the art for an LDO voltage regulatorcircuit that is capable of supporting regulation with both lesscapacitive and highly capacitive loads.

SUMMARY

In an embodiment, a low drop-out (LDO) voltage regulator circuitcomprises: a power transistor having a control terminal configured toreceive a control signal and an output terminal coupled to an outputnode; a current regulation loop configured to sense current flowingthrough the power transistor and modulate the control signal to causethe power transistor to output a constant current to the output node;and a voltage regulation loop configured to sense voltage at the outputnode and modulate the control signal to cause the power transistor todeliver current to the output node so that an output voltage at theoutput node is regulated. The current regulation loop comprises abipolar transistor having a first conduction terminal connected to thecontrol terminal of the power transistor and having a base terminaldriven by a signal that depends on a difference between the sensedcurrent flowing through the power transistor and a reference.

In an embodiment, a low drop-out (LDO) voltage regulator circuitcomprises: a power transistor having a control terminal and an outputterminal coupled to an output node; a current sensing circuit configuredto sense current flowing through the power transistor and generate afirst sense signal; a voltage sensing circuit configured to sensevoltage at the output node and generate a second sense signal; a firstdifferential amplifier having a first input configured to receive thefirst sense signal and a second input configured to receive a firstreference; a second differential amplifier having a first inputconfigured to receive the second sense signal and a second inputconfigured to receive a second reference; and a bipolar transistorhaving a control terminal configured to receive a signal output from thefirst differential amplifier and having a conduction terminal configuredto apply a signal to the control terminal of the power transistor;wherein a signal output from the second differential amplifier isapplied to the control terminal of the power transistor

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the embodiments, reference will now bemade by way of example only to the accompanying figures in which:

FIG. 1 is a block diagram of a conventional multi-mode low drop-out(LDO) voltage regulator circuit;

FIG. 2 is a block diagram of a multi-mode LDO voltage regulator circuit;

FIGS. 3A-3B illustrate circuit implementations for the differentialvoltage amplifier of the LDO voltage regulator circuit of FIG. 2;

FIGS. 4A-4B illustrate pole-zero plots for the LDO voltage regulatorwhen using the differential voltage amplifiers of FIGS. 3A-3B,respectively;

FIG. 5 illustrates a circuit implementation for the differential voltageamplifier of the LDO voltage regulator circuit of FIG. 2;

FIG. 6 shows an example application for the LDO circuit.

DETAILED DESCRIPTION

Reference is now made to FIG. 2 which shows a block diagram of amulti-mode low drop-out (LDO) voltage regulator circuit 100 configuredto support operation in one mode to enable a voltage regulation loop 114to regulate the delivery of a voltage at a load 116 and in another modeto enable a current regulation loop 118 to regulate the delivery of acurrent to the load 116. A power MOSFET device 124 has a source-draincurrent path coupled between an input voltage (Vin) node and the outputvoltage (Vout) node and configured to source a current to the outputnode. More specifically, the drain terminal of transistor 124 is coupledto the input voltage (Vin) node and a source terminal of transistor 124is coupled to the output voltage (Vout) node. The gate terminal of thetransistor 124 is configured to receive a gate voltage Vgate. The outputvoltage Vout at the load 116 is sensed using a resistive voltage dividerformed by the series connection of resistor R1 and resistor R2. The tapnode of the resistive voltage divider generates a feedback voltage Vfb.A current sensing circuit 132 senses the magnitude of the currentflowing in the source-drain path of the transistor 124 and generates afeedback voltage Cfb indicative of that sensed current magnitude.

The voltage regulation loop 114 controls the transistor 124 to delivercurrent to the load 116 in order to regulate the output voltage Vout ata level where the feedback voltage Vfb is substantially equal to avoltage regulation reference voltage Vref_vol. The gate voltage Vgatedriving the gate terminal of the transistor 124 is generated by adifferential voltage amplifier 130 having a first input configured toreceive the feedback voltage Vfb and a second input configured toreceive the voltage regulation reference voltage Vref_vol. The voltageregulation loop 114 thus modulates the gate voltage Vgate driving thegate terminal of the transistor 124 such that the current delivered bythe transistor 124 will regulate the output voltage Vout at the load116.

The current regulation loop 118 controls the transistor 124 to deliver aregulated constant current to the load 116. The gate voltage Vgatedriving the gate terminal of the transistor 124 is controlled by a PNPbipolar transistor 120 having an emitter terminal connected to the gateterminal of the transistor 124 and a collector terminal connected to apower supply reference (such as ground). A base terminal of bipolartransistor 120 receives a control signal 122 generated by a differentialvoltage amplifier 134 having a first input configured to receive thefeedback voltage Cfb and a second input configured to receive a currentregulation reference voltage Vref_cur. The differential voltageamplifier 134 and bipolar transistor 120 control conductivity of thetransistor 124 so that the feedback voltage Cfb is substantially equalto the current regulation reference voltage Vref_cur. The currentregulation loop 118 thus modulates the gate voltage Vgate driving thegate terminal of the transistor 124 such that a constant current of adesired magnitude is delivered by the transistor 124 to the load 116.

The bipolar transistor 120 functions as an analog switch to turn off thecurrent regulation loop 118. Advantageously, the bipolar transistor 120has a high base impedance which does not reduce the gain of the currentregulation loop 118. This assists in assuring a smooth transitionbetween modes. Furthermore, the bipolar transistor 120 features a highcurrent gain with low emitter impedance. This assists the operation ofthe current regulation loop 118 to clamp the gate voltage Vgate in thecontext of subsequently achieving the delivery of a constant current tothe load.

Constant current mode: Assume operation of the LDO voltage regulatorcircuit 100 where the output voltage Vout is at a level such that thefeedback voltage Vfb is much less than the voltage regulation referencevoltage Vref_vol. The voltage regulation loop 114 will control thetransistor 124 through the gate voltage Vgate to increase the magnitudeof the current being delivered to the load 116. This current is sensedby the current sensing circuit 132. When the feedback voltage Cfb risesdue to the increasing current to be substantially equal to the currentregulation reference voltage Vref_cur, the differential voltageamplifier 134 of the current regulation loop 118 triggers the turn on ofthe bipolar transistor 120 which clamps the gate voltage Vgate at avoltage level for the transistor 124 to deliver a constant current tothe load 116 having a desired magnitude. The active bipolar transistor120 with low emitter impedance reduces the voltage loop gain (byreducing the output impedance of the differential voltage amplifier 130)in order to ensure regulation by the current regulation loop 118. Itwill be noted that the differential voltage amplifier 130 can beimplemented with a current limiter or limited current source (as will beexplained elsewhere herein) in order to assist the bipolar transistor120 in clamping the gate voltage Vgate.

Transition from constant current mode to constant voltage mode: Assumenow that as a result of the constant charging operation the outputvoltage Vout has risen to a level such that the feedback voltage Vfb isclose to the voltage regulation reference voltage Vref_vol. Thedifferential voltage amplifier 130 responds by reducing the gate voltageVgate (i.e., reducing the voltage to a level that is below the clampvoltage imposed by the bipolar transistor 120 of the current regulationloop 118). As a result, there will be a corresponding reduction in thecurrent delivered by the transistor 124 to the load 116. This reductionin current is sensed by the current sensing circuit 132. Because of thehigh gain of the current regulation loop 118, the reduction in sensedcurrent is sufficient for the differential voltage amplifier 134 of thecurrent regulation loop 118 to turn off the bipolar transistor 120 andhand over control to the voltage regulation loop 114. It will be notedthat the differential voltage amplifier 134 may be configured to use alarge Miller capacitor in order to ensure loop stability and reduce loopbandwidth. The provision of a lower bandwidth for the current regulationloop 118 will ensure that the LDO circuit 100 does not inadvertentlyswitch back to constant current mode during the mode transition to moveto voltage regulation.

The circuit configuration for the differential voltage amplifier 130 canbe designed specifically for the type of load 116. For a standardcapacitive load (for example, 250 nF to 10 μF), the differential voltageamplifier 130 may have a circuit configuration as shown by FIG. 3A. Fora super-cap load (for example, 1 mF to 1 F), the differential voltageamplifier 130 may have a circuit configuration as shown by FIG. 3B.

Reference is now made to FIG. 3A which illustrates a circuitimplementation for the differential voltage amplifier 130. Adifferential input stage 132 has a first input configured to receive thefeedback voltage Vfb and a second input configured to receive the outputreference voltage Vref_out. An output of the differential input stage132 is applied to the input of a first gain stage 134. An output of thefirst gain stage 134 is applied to the input of a second gain stage 136.The output of the second gain stage 136 is fed back to the input of thesecond gain stage 136 by a feedback circuit 138 formed by the seriesconnection of a variable capacitor C1 and a variable resistor R4. Theoutput from the second gain stage 136 provides the gate voltage Vgate.An output driver circuit 140 of the second gain stage 136 is sourcebiased by a limited current produced by a current limiting resistor R3(or alternatively by a bias voltage limited current source circuit).

FIG. 4A is a plot of the poles and zeroes for the LDO circuit 100 withuse of the differential voltage amplifier 130 shown in FIG. 3A, where:

${zout} = {{- {\frac{1}{Rload*Cload}.}},}$with Rload being the resistance of the load 116 and Cload being thecapacitance of the load;

${{{adaptive}\mspace{14mu}{zero}} = \frac{1}{\left( {\frac{1}{gm2} - {R\; 4({Iout})}} \right)C\; 1}},$with gm2 being the transconductance of the second gain stage 136 andR4(Iout) being the variable resistance value which is dependent on Iout(the source-drain current in power transistor 124), where

${{R\; 4\;({Iout})} = {\frac{1}{K*{Iout}}{Cload}}},$with k being a design coefficient;

${{p1} = {- \frac{1}{r\; o\; 1\left( {gm2*{Rgate}} \right)C\; 1}}},$with ro1 being the output impedance of the first gain stage amplifier134 and Rgate being the input impedance of the power transistor 124;

${{p2} = {- \frac{gm2}{Cgp}}},$with Cgp being the gate capacitance of transistor 124;

${{p3} = {- \frac{1}{\frac{1}{gmp}*{Cload}}}},$with gmp being the transconductance of transistor 124;

UGB is the unity gain bandwidth;

reference 144 is at

$\frac{gm2}{C1}$which is the zero location in the absence of R4 (which is the adaptivezero); the arrow shows that the zero is shifted to the left half planeand becomes adaptive by means of the variable resistance R4;

jω is the imaginary axis; and

${{R4*C1} = {{\frac{1}{gmp}{Cload}} = {\frac{1}{k*{Iout}}{Cload}}}},$where this makes the zero adaptive to the moving pole p3 where both aredetermined by Iout (or gmp). As a consequence, the adaptive zero cancelsthe moving pole p3 in order to ensure loop stability.

Loop stability is ensured through use of a Miller capacitor to split thepoles p1 and p2. The adaptive zero is used to cancel the moving pole p3at the output, particularly in the case of the pole p3 moving into UGB.Zout is at a high frequency due to the small value of the loadcapacitance Cload. It is imperative to ensure that there is effectivelyonly one pole within UGB to achieve loop stability; a pair of poles andzero inside UGB cancel each other.

Reference is now made to FIG. 3B which illustrates a circuitimplementation for the differential voltage amplifier 130. Adifferential input stage 132 has a first input configured to receive thefeedback voltage Vfb and a second input configured to receive the outputreference voltage Vref_out. An output of the differential input stage132 is applied to the input of a first gain stage 154. An output of thefirst gain stage 154 is connected to a capacitor C2 provided to limitthe UGB and thereby ensure loop stability. The output of the first gainstage 154 is further applied to the input of a second gain stage 156.The output of the second gain stage 156 provides the gate voltage Vgate.The second gain stage 156 is formed by a first p-type MOSFET device 158configured as a source-follower transistor with a gate terminalconfigured to receive the output of the first gain stage 154, a drainterminal connected to a power supply reference voltage (for example,ground) and a source terminal connected to generate the gate voltageVgate. The source-follower transistor 158 is biased by a bias currentgenerated by a second p-type MOSFET device 160 operating as a currentsource with a source terminal connected to a power supply referencevoltage (for example, Vdd), a drain terminal connected to the sourceterminal of transistor 158 and a gate terminal receiving a bias voltageVb.

FIG. 4B is a plot of the poles and zeroes for the LDO circuit 100 withuse of the differential voltage amplifier 130 shown in FIG. 3B, where:

${{zout} = {- {\frac{1}{Rload*Cload}.}}},$with Rload being the resistance of the load 116 and Cload being thecapacitance of the load;

${{p\; 1^{\prime}} = {- \frac{1}{{ro}\; 1^{\prime}\left( {{C2} + {Cx}} \right)}}},$with ro1′ being the output impedance of the first gain stage amplifier154 and Cx being the total parasitic capacitance at the output of thefirst gain stage amplifier 154;

${{p\; 2^{\prime}} = {- \frac{{gm}\; 2^{\prime}}{Cgp}}},$with gm2′ being the transconductance of the source-follower transistor158 and Cgp being the gate capacitance of transistor 124; and

${{p3} = {- \frac{1}{\left( {\frac{1}{gmp} + {Rload}} \right)Cload}}},$with gmp being the transconductance of transistor 124;

UGB is the unity gain bandwidth; and

jω is the imaginary axis.

In this case, the pole p3 and the zero (zout) at the output both sit alow frequency within the unity gain bandwidth due to the largecapacitance (Cload) of the load 116. The zero (zout) is at a fixedfrequency that is determined by Rload and Cload. The pole p3 sits at aposition that changes with output current depending on change in gmp asshown by the expression for p3 above. Notwithstanding the outputcurrent, however, the pole p3 is always at a lower frequency than zout.

In order to stabilize the loop, the circuit introduces the pole p1′ at alocation close to, but not exceeding, the frequency of zout. Thisenables the loop gain to drop below 0 dB before entering the undesirablehigh frequency regime (associated with the pole p2′ and the parasiticpoles). The position of the pole p1′ can be adjusted by properlychoosing the capacitance of the capacitor C2 at the output of the firstgain stage amplifier 154. Thus, there are two poles and one zero withinthe unity gain bandwidth, and the relative positions are p3, zero andp1′ from the low frequency to the unity gain bandwidth.

Reference is now made to FIG. 5 which illustrates a circuitimplementation for the differential voltage amplifier 130. The circuit130 of FIG. 5 is a combination of the circuits of FIGS. 3A and 3B. Thecircuit of FIG. 3A, generally shown at reference 170, operates inresponse to the feedback voltage Vfb and the output reference voltageVref_out to generate a first voltage VlowC for application by thevoltage regulation loop 114 to the power transistor 124 as the gatevoltage Vgate. The circuit of FIG. 3B, generally shown at reference 172,operates in response to the feedback voltage Vfb and the outputreference voltage Vref_out to generate a second voltage VhighC forapplication by the voltage regulation loop 114 to the power transistor124 as the gate voltage Vgate. It will be noted that the circuits 170and 172 share a common differential input stage 132 providing output tofirst gain stages 134 and 154, respectively. An analog multiplexercircuit 172 has inputs that receive the first voltage VlowC and thesecond voltage VhighC and operates to select one of the input voltagesfor output as the gate voltage Vgate in response to a select signal(Select) that is, for example, an externally generated configurationsignal.

An advantage of the differential voltage amplifier 130 of FIG. 5 is thatthe LDO circuit 100 can be tuned for operation dependent on thecapacitance of the load 116. In the case of a relatively lower loadcapacitance (for example, 250 nF to 10 μF), the select signal (Select)can control the multiplexer 174 to pass the first voltage VlowCgenerated by circuit 170 as the gate voltage Vgate. In the case of arelatively higher load capacitance (for example, 1 mF to 1 F), theselect signal (Select) can control the multiplexer 174 to pass thesecond voltage VhighC generated by circuit 172 as the gate voltageVgate.

Reference is now made to FIG. 6 which shows an example application forthe LDO circuit 100. A mobile device, such as a cellular telephone, istypically powered by a battery which needs to be recharged. It isdesired for such a mobile device to support a wireless charging feature.To accomplish this function, the mobile device includes an antennacoupled to a rectifier circuit which converts the AC signal received bythe to a DC voltage Vin. The LDO circuit receives the voltage Vin andgenerates a current for charging the battery to the voltage level Vout.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims.

What is claimed is:
 1. A low drop-out (LDO) voltage regulator circuit,comprising: a power transistor having a control terminal configured toreceive a control signal and an output terminal coupled to an outputnode; a current regulation loop configured to sense current flowingthrough the power transistor and modulate the control signal to causethe power transistor to output a constant current to the output node;and a voltage regulation loop configured to sense voltage at the outputnode and modulate the control signal to cause the power transistor todeliver current to the output node so that an output voltage at theoutput node is regulated; wherein the current regulation loop comprisesa control transistor having a first conduction terminal connected to thecontrol terminal of the power transistor and having a control terminaldriven by a signal that depends on a difference between the sensedcurrent flowing through the power transistor and a reference; whereinthe voltage regulation loop comprises: a differential input circuithaving a first input configured to receive the voltage sensed at theoutput node and a second input configured to receive a voltageregulation reference voltage; and a gain circuit having an input coupledto an output of the differential input circuit and an output configuredto generate the control signal for application to the control terminalof the power transistor; wherein the gain circuit includes a feedbackcircuit having a resistor and a capacitor coupled in series, wherein aresistance of the resistor and a capacitance of the capacitor set a zerofor canceling a pole at the output node.
 2. The circuit of claim 1,wherein the first conduction terminal of the bipolar transistor is anemitter terminal.
 3. The circuit of claim 1, wherein the powertransistor is a MOSFET device.
 4. The circuit of claim 3, wherein theoutput terminal of the power transistor MOSFET device is a sourceterminal.
 5. The circuit of claim 1, wherein the pole at the output nodeis set by a load circuit connected to the output node.
 6. The circuit ofclaim 1, wherein the resistor is a variable resistor and the capacitoris a variable capacitor.
 7. The circuit of claim 1, wherein the gaincircuit comprises: a first gain stage having an input coupled to theoutput of the differential input circuit; and a second gain stage havingan input coupled to an output of the first gain stage; wherein thefeedback circuit is coupled between the output of the second gain stageand the input of the second gain stage.
 8. The circuit of claim 7,wherein the second gain stage includes an output drive circuit having acurrent limiting source circuit.
 9. The circuit of claim 8, wherein thecurrent limiting source circuit is a current limiting resistor.
 10. Thecircuit of claim 1, wherein the control transistor is a bipolartransistor and wherein the power transistor is a MOSFET device.
 11. Alow drop-out (LDO) voltage regulator circuit, comprising: a powertransistor having a control terminal configured to receive a controlsignal and an output terminal coupled to an output node; a currentregulation loop configured to sense current flowing through the powertransistor and modulate the control signal to cause the power transistorto output a constant current to the output node; and a voltageregulation loop configured to sense voltage at the output node andmodulate the control signal to cause the power transistor to delivercurrent to the output node so that an output voltage at the output nodeis regulated; wherein the current regulation loop comprises a controltransistor having a first conduction terminal connected to the controlterminal of the power transistor and having a control terminal driven bya signal that depends on a difference between the sensed current flowingthrough the power transistor and a reference; wherein the voltageregulation loop comprises: a differential input circuit having a firstinput configured to receive the voltage sensed at the output node and asecond input configured to receive a voltage regulation referencevoltage; and a gain circuit having an input coupled to an output of thedifferential input circuit and an output configured to generate thecontrol signal for application to the control terminal of the powertransistor; wherein the gain circuit includes a capacitor having acapacitance to set a pole at a frequency that does not exceed a zero atthe output node.
 12. The circuit of claim 11, wherein the zero at theoutput node is set by a load circuit connected to the output node. 13.The circuit of claim 11, wherein the gain circuit comprises: a firstgain stage having an input coupled to the output of the differentialinput circuit; and a second gain stage having an input coupled to anoutput of the first gain stage; wherein the capacitor has a firstterminal connected to the output of the first gain stage and a secondterminal connected to a power supply reference voltage.
 14. The circuitof claim 13, wherein the power supply reference voltage is ground. 15.The circuit of claim 13, wherein the second gain stage comprises asource-follower transistor.
 16. The circuit of claim 15, wherein thesecond gain stage further comprises a current source configured to biasthe source-follower transistor.
 17. The circuit of claim 11, wherein thefirst conduction terminal of the bipolar transistor is an emitterterminal.
 18. The circuit of claim 11, wherein the power transistor is aMOSFET device.
 19. The circuit of claim 18, wherein the output terminalof the power transistor MOSFET device is a source terminal.
 20. Thecircuit of claim 11, wherein the control transistor is a bipolartransistor and wherein the power transistor is a MOSFET device.
 21. Alow drop-out (LDO) voltage regulator circuit, comprising: a powertransistor having a control terminal configured to receive a controlsignal and an output terminal coupled to an output node; a currentregulation loop configured to sense current flowing through the powertransistor and modulate the control signal to cause the power transistorto output a constant current to the output node; and a voltageregulation loop configured to sense voltage at the output node andmodulate the control signal to cause the power transistor to delivercurrent to the output node so that an output voltage at the output nodeis regulated; wherein the current regulation loop comprises a controltransistor having a first conduction terminal connected to the controlterminal of the power transistor and having a control terminal driven bya signal that depends on a difference between the sensed current flowingthrough the power transistor and a reference; wherein the voltageregulation loop comprises: a differential input circuit having a firstinput configured to receive the voltage sensed at the output node and asecond input configured to receive a voltage regulation referencevoltage; a first amplifier circuit having an input coupled to an outputof the differential input circuit, the first amplifier circuitconfigured to generate a first control signal; a second amplifiercircuit having an input coupled to the output of the differential inputcircuit, the second amplifier circuit configured to generate a secondcontrol signal; and an analog multiplexer circuit having a first inputconfigured to receive the first control signal and a second inputconfigured to receive the second control signal, wherein the analogmultiplexer circuit selectively passes one of the first and secondcontrol signals as the control signal applied to the control terminal ofthe power transistor in response to a select signal.
 22. The circuit ofclaim 21, wherein the first amplifier circuit comprises: a gain circuithaving an input coupled to an output of the differential input circuitand an output configured to generate the first control signal; whereinthe gain circuit includes a feedback circuit having a resistor and acapacitor coupled in series, wherein a resistance of the resistor and acapacitance of the capacitor set a zero for canceling a pole at theoutput node.
 23. The circuit of claim 22, wherein the gain circuitcomprises: a first gain stage having an input coupled to the output ofthe differential input circuit; and a second gain stage having an inputcoupled to an output of the first gain stage; wherein the feedbackcircuit is coupled between the output of the second gain stage and theinput of the second gain stage.
 24. The circuit of claim 23, wherein thesecond gain stage includes an output drive circuit having a currentlimiting source circuit.
 25. The circuit of claim 21, wherein the secondamplifier circuit comprises: a gain circuit having an input coupled toan output of the differential input circuit and an output configured togenerate the second control signal; wherein the gain circuit includes acapacitor having a capacitance to set a pole at a frequency that doesnot exceed a zero at the output node.
 26. The circuit of claim 25,wherein the gain circuit comprises: a first gain stage having an inputcoupled to the output of the differential input circuit; and a secondgain stage having an input coupled to an output of the first gain stage;wherein the capacitor has a first terminal connected to the output ofthe first gain stage and a second terminal connected to a power supplyreference voltage.
 27. The circuit of claim 26, wherein the second gainstage comprises a source-follower transistor.
 28. The circuit of claim21, wherein the first conduction terminal of the bipolar transistor isan emitter terminal.
 29. The circuit of claim 21, wherein the powertransistor is a MOSFET device.
 30. The circuit of claim 29, wherein theoutput terminal of the power transistor MOSFET device is a sourceterminal.
 31. The circuit of claim 21, wherein the control transistor isa bipolar transistor and wherein the power transistor is a MOSFETdevice.